tìm các giới hạn sau:
a; \(\lim\limits_{x\rightarrow1}\frac{2x^4-5x^3+3x^2+1}{3x^4-8x^3+6x^2-1}\)
b; \(\lim\limits_{x\rightarrow1}\frac{x^3-3x^2+2}{x^4-4x+3}\)
c; \(\lim\limits_{x\rightarrow1}\frac{x^3-2x-1}{x^5-2x-1}\)
d; \(\lim\limits_{x\rightarrow-1}\frac{\left(x+2\right)^2-1}{x^2-1}\)